Endothelium, for example.

Endothelial nitric oxide production contributes at many levels to cardiovascular efficiency (considered as tissue perfusion relative to cardiac work). Endothelial dysfunction is found in many conditions, including all known risk factors for atheroma, and is usually generalised, when present, involving microvessels as well as large arteries, impairing cardiovascular efficiency and potentially initiating atheroma. Loss of flow-related dilation is a manifestation of endothelial dysfunction and can be measured non-invasively, thereby providing a potential marker of reversible atherogenic susceptibility.